Advances in polymerization and catalysis have resulted in the ability to produce many new polymers having improved physical and chemical properties useful in a wide variety of superior products and applications. With the development of new catalysts the choice of polymerization process (solution, slurry, high pressure or gas phase) for producing a particular polymer have been greatly expanded. Also, advances in polymerization technology has provided more efficient, highly productive and economically enhanced processes. Especially illustrative of these advances is the development of the technology field utilizing metallocene catalyst systems.
As with a new technology field, particularly in the polyolefins industry, a small savings in cost often determines whether a commercial endeavor is even feasible. This aspect in the metallocene technology field is evident by the number of participants in the industry looking for new ways to reduce cost. In particular, there has been tremendous focus in the industry on developing new and improved metallocene catalyst systems. Some have focused on designing the catalyst systems to produce new polymers, others on improved operability, and many more on improving catalyst productivity. The productivity of a catalyst, that is the amount of polymer produced per gram of the catalyst, usually is the key economic factor that can make or break a new commercial development in the polyolefin industry. Reactor operability—lack of fouling and sheeting, etc.—of the polymerization reactor is also a major concern for polyolefin producers. Reducing the occurrence of reactor fouling has commercial benefits in reduced down time for the reactor and improved output of polyolefin resin, as well as higher quality resin.
From the early stages in the metallocene technology field, beginning with the discovery of the utility of alumoxane as a cocatalyst in the early 1980's, to the discovery of substitutions on the bulky ligands of the metallocene compounds, through the development of non-coordinating anions, and today with the ever increasing number of new metallocene bulky ligand compounds, catalyst productivity has been a primary focus.
Evidence of this can be seen in this subset of the art discussing various metallocene catalyst compounds and catalyst systems described in U.S. Pat. Nos. 4,530,914, 4,542,199, 4,769,510, 4,871,705, 4,937,299, 5,017,714, 5,055,438, 5,096,867, 5,130,030, 5,120,867, 5,124,418, 5,198,401, 5,210,352, 5,229,478, 5,264,405, 5,278,264, 5,278,119, 5,304,614, 5,324,800, 5,347,025, 5,350,723, 5,384,299, 5,391,790, 5,391,789, 5,399,636, 5,408,017, 5,491,207, 5,455,366, 5,534,473, 5,539,124, 5,554,775, 5,621,126, 5,684,098, 5,693,730, 5,698,634, 5,710,297, 5,712,354, 5,714,427, 5,714,555, 5,728,641, 5,728,839, 5,753,577, 5,767,209, 5,770,753, 5,770,664 and 5,814,574, European Patent Nos. EP-A-0 591 756, EP-A-0 520 732, EP-A-0 420 436, EP-B1 0 485 822, EP-B1 0 485 823, EP-A2-0 743 324 and EP-B1 0 518 092 and PCT Publication Nos. WO 91/04257, WO 92/00333, WO 93/08221, WO 93/08199, WO 94/01471, WO 96/20233, WO 97/15582, WO 97/19959, WO 97/46567, WO 98/01455, WO 98/06759 and WO 98/011144.
There are many more examples in the metallocene art. However, there is a small subset that discuss the importance of the leaving group, the ligand capable of being abstracted and rendering the metallocene catalyst system capable of polymerizing olefins. Some in art discuss using chloride or methyl leaving groups, for example U.S. Pat. Nos. 4,542,199 and 4,404,344 respectively.
Much of the metallocene art discuss the use generally of halogens as leaving groups. For example, EP-A2 0 200 351 mentions in a laundry list of possibilities, a few compounds having fluoride leaving groups, as does EP-A1 0 705 849. However, although halogens are typically discussed in much of the art, the predominant focus has been on chlorine as a leaving group.
There are some disclosures and exemplifications of metallocene compounds having fluoride groups in the art, for example:
E. F. Murphy, et al., “Synthesis and spectroscopic characterization of a series of substituted cyclopentadienyl Group 4 fluorides; crystal structure of the acetlacetonato complex [(acac)2(η5-C5Me5)Zr(μ-F)SnMe3Cl]”, DALTON 1983 (1996), describes the synthesis of some mono- and di-substituted cyclopentadienyl Group 4 fluoride compounds.
Herzog, et al., “Reactions of (η5-C5Me5)ZrF3, (η5-C5Me4Et)ZrF3, (η5-C5M45)2ZrF2, (η5-C5Me5)HfF3, and (η5-C5Me5)TaF4 with AlMe3. Structure of the First Hafnium-Aluminum-Carbon Cluster”, 15 ORGANOMETALLICS 909-917 (1996), describes the reactions of various compounds having fluoride leaving groups with an aluminum compound.
F. Garbassi, et al., JOURNAL OF MOLECULAR CATALYSIS A: CHEMICAL 101 199-209 (1995) illustrates the binding energy of various leaving groups on zirconium compounds. In particular this article shows that a catalyst system of bis(cyclopentadienyl)zirconium dichloride in the polymerization of ethylene is more active than the di-fluoride analog.
PCT publication WO 97/07141 describes a number of metallocene compounds with fluoride leaving groups. This publication exemplifies their use with methylalumoxane in the polymerization of styrene and shows a single bis(cyclopentadienyl)titanium mono-fluoride having a very low productivity. Also, Kaminsky, et al., “Fluorinated Half-Sandwich Complexes as Catalysts in Syndiospecific Styrene Polymerization”, 30(25) MACROMOLECULES 1997 describes that unbridged mono-cyclopentadienyl titanium trifluoride catalysts have a higher activity than the chlorinated compounds in the polymerization of styrene in the temperature range of from 10° C. to 70° C.
German publication DE 43 32 009 A1 describes a process for making organometallic fluorides by reacting an organometallic halide with tin fluoride. This publication appears to show that an unsupported catalyst system of methylalumoxane and a bis(pentamethylcyclopentadienyl)zirconium dichloride has a lower homopolyethylene productivity compared with double the amount of the difluoride at 70° C.
Considering the discussion above there is still a need for higher productivity catalyst systems capable of providing the efficiencies necessary for implementing commercial polyolefin process. Further, it has been found, especially in gas phase fluidized bed processes, that reactor performance (presence or absence of reactor fouling, sheeting, etc.) is an issue when using supported metallocene catalysts. Secondary additives or support “surface modifiers” are often used to reduce fouling and hence improve commercial performance of the reactor. Addition of these surface modifiers, however, adds cost and complexity to the polymerization process. Thus, it would be highly advantageous to have a polymerization process and catalyst system capable of producing polyolefins with improved catalyst productivities and reactor performance.